Abstract: The high intense electron beam interacts with a high Z target, producing the X-rays for flash radiography. The focal spot size of the X-ray source is a critical parameter which degrades resolution in the flash radiography. This paper describes a slit which is used to measure the focal spot size. The X-ray spot image is obtained through the slit. The line spread function(LSF) and modulation transfer function(MTF) of X-ray spot are obtained from the slit image data, and the X-ray spot size is determined by the MTF. The measured X-ray spot size of the 12 MeV linear induction accelerator(LIA) is 3.2 mm. Different focal lengths are used, and the relevant X-ray spots size is measured for tuning the accelerator.